This invention relates to investigations of earth formations, and more particularly elates to nuclear magnetic resonance (NMR) logging of earth formations.
NMR has been a common laboratory technique for over forty years and has become an important tool in formation evaluation. General background of NMR well logging can be found, for example, in U.S. Pat. No. 5,023,551 to Kleinberg et al., which is assigned to the same assignee as the present invention and herein incorporated by reference in its entirety.
NMR relies upon the fact that the nuclei of many chemical elements have angular momentum (xe2x80x9cspinxe2x80x9d) and a magnetic moment. In an externally applied static magnetic field, the spins of nuclei align themselves along the direction of the static field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g., an RF pulse) that tips the spins away from the static field direction. The angle through which the spins are tipped is given by xcex8=xcex3B1tp/2, where xcex3 is the gyromagnetic ratio, B1 is the linearly polarized oscillating field strength, and tp is the duration of the pulse. Tipping pulses of ninety and one hundred eighty degrees are most common.
After tipping, two things occur simultaneously. First, the spins precess around the direction of the static field at the Larmor frequency, given by xcfx890=xcex3B0, where B0 is the strength of the static field and xcex3 is the gyromagnetic ratio. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so, for example, in a static field of 235 Gauss, the hydrogen spins would precess at a frequency of 1 MHz. Second, the spins return to the equilibrium direction according to a decay time, T1, which is known as the spin-lattice relaxation time. Because this spin-lattice relaxation occurs along the equilibrium direction, T1, is also referred to as the longitudinal relaxation time constant.
Also associated with the spin of molecular nuclei is a second relaxation time, T2, called the spin-spin relaxation time. At the end of a ninety-degree tipping pulse, all the spins are pointed in a common direction perpendicular, or transverse, to the static field, and they all precess at the Larmor frequency. However, because of small fluctuations in the static field induced by other spins or paramagnetic impurities, the spins precess at slightly different frequencies, and the transverse magnetization dephases with a time constant T2, which is also referred to as the transverse relaxation time constant.
A standard technique for measuring T2, both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, an initial pulse tips the spins into the transverse plane and causes the spins to start precessing. Then, a one hundred eighty-degree pulse is applied that keeps the spins in the measurement plane, but causes the spins, which are dephasing in the transverse plane, to reverse direction and to refocus. By repeatedly reversing the spins using a series of one hundred eighty degree pulses, a series of xe2x80x9cspin echoesxe2x80x9d appear. The train of echoes is measured and processed to determine the irreversible dephasing time constant, T2. In well logging applications, the detected spin echoes have been used to extract oilfield parameters such as porosity, pore size distribution, and oil viscosity.
The invention acquires and analyzes a different type of magnetic resonance signal than is typically detected and analyzed in current nuclear magnetic resonance well logging methods. In some embodiments, this other signal is generated, acquired and analyzed along with the spin echoes that are generated in nuclear magnetic resonance logging methods based on the CPMG sequence. This other signal has been recognized by the inventors to be a steady state free precession (SSFP) signal. Thus, according to the invention, a method of evaluating an earth formation includes introducing a nuclear magnetic resonance logging tool into a borehole that traverses the earth formation to apply a sequence of magnetic pulses to a region of investigation within the earth formation. The nuclear magnetic resonance tool detects a SSFP signal from the region, and the SSFP signal is analyzed to extract information about the region of investigation.
Further details and features of the invention will become more readily apparent from the detailed description that follows.